1. Technical Field
The present invention generally relates to novel methods for preparing non-monolithic polymeric resin materials via emulsion or suspension processes by employing an acid saturated secondary phase, and wherein said material is carbonized resulting in unexpected improvement in electrochemical performance. The methods herein can also be employed generally to improve emulsion and suspension polymerization processes by improved control of diffusion of acidic and basic species between the polymer and secondary phases.
2. Description of the Related Art
Activated carbon is commonly employed in electrical storage and distribution devices. The surface area, conductivity and porosity of activated carbon allows for the design of electrical devices having desirable electrochemical performance. Electric double-layer capacitors (EDLCs or “ultracapacitors”) are an example of such devices. EDLCs often have electrodes prepared from an activated carbon material and a suitable electrolyte, and have an extremely high energy density compared to more common capacitors. Typical uses for EDLCs include energy storage and distribution in devices requiring short bursts of power for data transmissions, or peak-power functions such as wireless modems, mobile phones, digital cameras and other hand-held electronic devices. EDLCs are also commonly used in electric vehicles such as electric cars, trains, buses and the like.
Batteries are another common energy storage and distribution device which often contain an activated carbon material (e.g., as anode material, current collector, or conductivity enhancer). For example, lithium/carbon batteries having a carbonaceous anode intercalated with lithium represent a promising energy storage device. Other types of carbon-containing batteries include lithium air batteries, which use porous carbon as the current collector for the air electrode, and lead acid batteries which often include carbon additives in either the anode or cathode. Batteries are employed in any number of electronic devices requiring low current density electrical power (as compared to an EDLC's high current density).
One known limitation of EDLCs and carbon-based batteries is decreased performance at high-temperature, high voltage operation, repeated charge/discharge cycles and/or upon aging. This decreased performance has been attributed, at least in part, to electrolyte impurity or impurities in the carbon electrode itself, causing breakdown of the electrode at the electrolyte/electrode interface. Thus, it has been suggested that EDLCs and/or batteries comprising electrodes prepared from higher purity carbon materials could be operated at higher voltages and for longer periods of time at higher temperatures than existing devices.
In addition to purity, another known limitation of carbon-containing electrical devices is the pore structure of the activated carbon itself. While activated carbon materials typically comprise high porosity, the pore size distribution is not optimized for use in electrical energy storage and distribution devices. Such optimization may include a blend of both micropores and mesopores. Additionally in some applications a high surface area carbon may be desirable, while in others a low surface are material is preferred. Idealized pore size distributions can maximize performance attributes including but not limited to, increased ion mobility (i.e., lower resistance), increased power density, improved volumetric capacitance, increased cycle life efficiency of devices prepared from the optimized carbon materials.
One common method for producing carbon materials is to pyrolyze an existing carbon-containing material (e.g., coconut fibers or tire rubber). This results in a char with relatively low surface area which can subsequently be over-activated to produce a material with the surface area and porosity necessary for the desired application. Such an approach is inherently limited by the existing structure of the precursor material, and typically results in a carbon material having an unoptimized pore structure and an ash content (e.g., metal impurities) of 1% or higher.
Activated carbon materials can also be prepared by chemical activation. For example, treatment of a carbon-containing material with an acid, base or salt (e.g., phosphoric acid, potassium hydroxide, sodium hydroxide, zinc chloride, etc.) followed by heating results in an activated carbon material. However, such chemical activation results in relatively high levels of undesired non-carbon elements (even after washing procedures), that in turn impair the carbon performance in electrical devices.
Another approach for producing high surface area activated carbon materials is to prepare a synthetic polymer from carbon-containing organic building blocks (e.g., a polymer gel). As with the existing organic materials, the synthetically prepared polymers are pyrolyzed and activated to produce an activated carbon material. In contrast to the traditional approach described above, the intrinsic porosity of the synthetically prepared polymer results in higher process yields because less material is lost during the activation step. Methods for producing activated carbon from synthetic polymer, for example production of carbon aerogels, xerogels, and cryogels on the laboratory scale are known in the art.
Although such methods may be applicable in laboratory or small-scale settings, preparation of large quantities of carbon materials via synthetic polymers may be limited at large scales. The monolithic nature of polymer gels are difficult and expensive to produce and convert into the end product, i.e., aerogel, xerogel, or cryogel. Due to the monolith's large size and low thermal conductivity a significant amount of energy, time, and specialized equipment is required in order to polymerize the monomer component that makes up the monolith structure. Additionally, due to the uneven heating of the monolithic polymer gel as heat is transferred from the outside to the inside thereof, heterogeneous physical differences in the monolithic polymer are formed which can negatively impact the performance of the carbon material produced therefrom. This uneven heating combined with the exothermic nature of polymerization results in difficulty in controlling the extent of polymerization, with the consequence of reduced ability to fine tune the gel pore structure (and pore structure of the carbon material produced therefrom). Furthermore, large monolithic polymer gels are difficult to work with (e.g., transfer from one vessel to another) and in order to facilitate processed into carbon require post-polymerization particle size reduction (e.g., grinding, milling, etc.), which results in increased labor, capital and production costs, and processing steps and time.
There is a need, therefore, for improved methods for making polymer particles in gel form, in order to further facilitate cost-effective and tunable methods for preparing high purity and high performance carbon materials for use in electrical energy storage devices. The present invention meets this need by providing an improved method for producing non-monolithic sol gel polymer that unexpectedly results in electrochemical improvement for activated carbon produced from same.